Bump version to 0.8.0 for incompatible changes #717

Job	Run time
Clippy (Nightly)	41s
Clippy	1m 2s
Build+test (stable, ubuntu-latest)	1m 43s
Check for documentation errors	15s
Build+test (beta, ubuntu-latest)	1m 39s
Format	8s
Build+test (nightly, ubuntu-latest)	1m 38s
Check minimum versions of direct dependencies	14s
Build+test (macos-latest, stable)	1m 48s
msrv (ubuntu-latest)	2m 38s
Build+test (windows-latest, stable)	4m 47s
Check semver compatibility	15s
msrv (macos-latest)	2m 29s
msrv (windows-latest)	3m 12s
	22m 29s

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